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Cytokinins (CK) are a class of plant growth substances (plant hormones) that promote cell division. They are primarily involved in cell growth, differentiation, and other physiological processes. Their effects were first discovered through the use of coconut milk in the 1940s by a scientist at the University of Wisconsin-Madison named Folke Skoog.citation needed
There are two types of cytokinins: adenine-type cytokinins represented by kinetin, zeatin and 6-benzylaminopurine, as well as phenylurea-type cytokinins like diphenylurea or thidiazuron (TDZ). The adenine-type cytokinins are synthesised in stems, leaves and roots, which is the major site.citation needed Cambium and possibly other actively dividing tissues are also sites of cytokinin biosynthesis.[1] There is no evidence that the phenylurea cytokinins occur naturally in plant tissues.[2] Cytokinins are involved in both local and long distance signalling, the latter of which involves the same in planta transport mechanism as used for transport of purines and nucleosides.[3]
Cytokinins are involved in many plant processes, including cell division, shoot and root morphogenesis, chloroplast maturation, cell enlargement, auxiliary bud release and senescence.[4] The ratio of auxin to cytokinin is crucial during cell division and the differentiation of plant tissues. Auxin is known to regulate the biosynthesis of cytokinin.[5]
Biosynthesis
Adenosine phosphate-isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesis of isoprene cytokinins. It may use ATP, ADP or AMP as substrates and may use dimethylallyl diphosphate (DMAPP) or hydroxymethylbutenyl diphosphate (HMBDP) as prenyl donors.[6] This reaction is the rate limiting step in cytokinin biosynthesis. DMAPP and HMBDP used in cytokinin biosynthesis are produced by the methylerythritol phosphate pathway (MEP).[6]
Cytokinins can also be produced by recycled tRNAs in plants and bacteria.[6][7] tRNAs with anticodons that start with a uridine and carrying an already prenylated adenosine adjacent to the anticodon release on degradation the adenosine as a cytokinin.[6] The prenylation of these adenines is carried out by tRNA-isopentenyltransferase.[7]
References
- ^ Chen, C. et al. 1985. Localization of Cytokinin Biosynthetic Sites in Pea Plants and Carrot Roots. Plant Physiology 78:510–513.
- ^ Mok, DWS and Mok, MC. 2001. Cytokinin metabolism and action. Annual Review of Plant Physiology and Plant Molecular Biology 52: 89-118
- ^ Sakakibara, H. 2006. Cytokinins: Activity, Biosynthesis, and Translocation. Annual Review of Plant Biology 57: 431-449
- ^ Kieber JJ (2002 Cytokinins. In CR Somerville, EM Meyerowitz, eds, [www.aspb.org/publications/arabidopsis/ The Arabidopsis Book]. American Society of Plant Biologists, Rockville, MD, doi: 10.1199/tab.0009
- ^ Nordström, A. 2004. Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana: A factor of potential importance for auxin–cytokinin-regulated development. PNAS 101:8039–8044
- ^ a b c d Ildoo Hwang, Hitoshi Sakakibara (2006) Cytokinin biosynthesis and perception Physiologia Plantarum 126 (4), 528–538
- ^ a b Kaori Miyawaki, Miho Matsumoto-Kitano, Tatsuo Kakimoto (2004) Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate The Plant Journal 37 (1), 128–138
External links
- Chapter 21 of Plant Physiology Book: Cytokinins: Regulators of Cell Division
- Plant Physiology:Cytokinin
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- This page was last modified on 13 October 2008, at 03:39.
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